The present invention relates generally to multichannel record disc reproducing apparatus, and more particularly to such an apparatus for reproducing and demodulating a picked up signal after effectively reducing or removing abnormal noise therein.
A multichannel record disc is recorded on a multichannel signal produced by multiplexing a direct wave signal and an angle-modulated wave signal. This multichannel signal is picked up, and the angle-modulated wave signal is demodulated by a demodulator which, in general, uses a phase locked loop (PLL) circuit.
As is known, the width of the lock range of a PLL circuit varies in accordance with the level of the input angle-modulated wave signal. Furthermore, the frequency band width of the output demodulated signal of a PLL circuit is determined by the lock range of the PLL circuit. Accordingly, when a high level angle-modulated wave signal is introduced as input into the PLL circuit, the width of the demodulated frequency band is wide. Whereas, when a low level angle-modulated wave signal appears, the demodulated frequency band width is narrow. In this manner, the lock range of a PLL circuit varies automatically with the level of the input angle-modulated wave signal. Also, in accordance with this variation, the demodulated frequency band width of the output signal varies. Therefore, a PLL circuit having a characteristic of this nature is suitable for use in the demodulation circuit of a multichannel record disc reproducing apparatus.
In a multichannel record disc, the frequency band of the direct wave signal is in the range from 30 Hz to 15 KHz, whereas the frequency band of the angle-modulated wave signal is in the range from 20 KHz to 45 KHz, for example. Accordingly, the bands of the two signals are relatively close to each other. Therefore, if a signal has a large component, particularly in the high-frequency part of the direct wave signal, higher harmonics become mixed in the band of the angle-modulated wave signal. When these mixed higher harmonic components are vectorially added to the angle-modulated wave signal, there may be an extinction or lack of the angle-modulated signal or an extreme distortion of the angle-modulated signal.
When a signal temporarily disappears from the input angle-modulated wave signal of the PLL circuit, the PLL circuit unlocks. Furthermore, when distortion is present in the input angle-modulated wave signal, the PLL circuit may lock onto the distortion. In either case, abnormal noise of pulse form is generated in the output demodulated signal.
Accordingly, in order to eliminate or reduce this abnormal noise, the systems of U.S. Pat. Nos. 3,896,272 and 3,991,283 have been adopted. In each of these systems, a synchronous detector is supplied with the output of a voltage-controlled oscillator connected within the loop of the PLL circuit used with the input angle-modulated wave signal. Abrupt variations are detected in the phase appearing in the output of the voltage-controlled oscillator. In accordance with this detection signal and through the use of a control signal, either the loop gain of the PLL circuit or a muting circuit is controlled thereby to reduce or eliminate the generation of the abnormal noise.
However, this known system does not produce an output from the synchronous detector only when an abnormal noise is generated in the demodulated signal from the PLL circuit, as where the PLL circuit unlocks or the PLL circuit locks onto the distortion in the angle-modulated wave signal. Although abnormal noise is not produced in the demodulated output signal from the PLL circuit, a distorted output is sometimes produced at the synchronous detector so that a great phase change occurs abruptly in the angle-modulated wave signal. In such a case, the loop gain control circuit or a muting circuit operates erroneously.
More specifically, consider the demodulation problems when the angle-modulated wave signal to be demodulated is a carrier modulated by a signal having a frequency component in a middle and low frequency ranges, such as, for example, a trumpet sound. Such a waveform may be a train of pulses with steep rising leading edges and high values. Large phase changes occur abruptly in this angle-modulated wave signal because the frequency deviation is large. When the loop gain of the PLL circuit is set at a large value, the lock range is wide, and noise is readily generated. For this reason, the loop gain of the PLL circuit is set at a relatively small value. Although the resulting lock range covers the frequency band of the angle-modulated wave, it will not be remarkably wider than this.
Accordingly, when large phase changes occur abruptly in an angle-modulated wave signal supplied to the PLL circuit, the phase of the output from the voltage-controlled oscillator within the PLL circuit has a great deviation. An output is produced from the synchronous detector. However, in this case, the PLL circuit continues its normal demodulation operation and produces a normal demodulated output without unlocking because of the so-called flywheel effect. For this reason, even if the PLL circuit produces a normal demodulated output, a control signal output is produced from the synchronous detector. The loop gain of the PLL circuit, the muting circuit operation, and the like, are unnecessarily controlled.